The present invention relates to a cathode ray tube and, more particularly, to a cathode ray tube which prevents the reflection of external light on a glass panel portion of the tube envelope, so as to raise the display contrast and prevent the formation of an electrostatic charge on the screen.
In a cathode ray tube to be used in a TV receiver or a personal computer monitor, a tube envelope in the form of a glass vacuum enclosure is used, which comprises a glass panel having a screen or an image display screen formed thereon, a neck portion housing electron guns and a funnel portion connecting the glass panel and the neck portion. A phosphor film representing the screen formed on the inner face of the glass panel is excited with modulated electron beams emitted from the electron guns to display a desired image.
FIG. 11 is a section view for explaining the structure of a shadow mask color cathode ray tube, which represents one example of a cathode ray tube with which the present invention is concerned. In FIG. 11, reference numeral 1 designates a glass panel portion; numeral 2 denotes a neck portion; numeral 3 denotes a funnel portion; numeral 4 denotes a phosphor screen; numeral 5 denotes a shadow mask; numeral 6 denotes a mask frame; numeral 7 denotes mask support mechanism; numeral 8 denotes support pins; numeral 9 denotes an inner magnetic shield; numeral 10 denotes anode button; numeral 11 denotes an internal conductive coating; numeral 12 denotes a deflector: numeral 13 denotes electron guns; and numeral 14 denotes electron beams (red, green and blue). In the cathode ray tube shown in FIG. 1 a tube envelope in the form of a vacuum enclosure is constructed of the glass panel portion 1 on which the screen (phosphor film 4) is formed, the neck portion 2 housing the electron guns and the funnel portion 3 connecting the glass panel portion and the neck portion. The inner wall surface of this vacuum enclosure is coated with the internal conductive coating 11 for supplying a high anode voltage, applied to the anode button 10, to the screen and the electron guns.
The shadow mask 5 is welded to the mask frame 6 and is suspended by the support mechanism 7 from the support pins 8, which are buried in the inner wall of the skirt portion of the glass panel portion 1, so that the shadow mask is held at a predetermined small spacing from the phosphor screen 4 formed on the inner face of the glass panel portion 1.
The inner magnetic shield 9 is provided for shielding the image display from the bad influences of external magnetic fields, such as the earth""s magnetism, upon the electron beams 14 and is welded to and held by the mask frame 6.
On the neck portion side of the funnel portion 3, there is mounted the deflection coils 12 for establishing a horizontal magnetic field and a vertical magnetic field within the tube envelope, so that the three modulated electron beams emitted from the electron guns 13 are deflected in the horizontal direction and in the vertical direction to scan the phosphor film two-dimensionally and thereby to display a desired image.
Generally, this cathode ray tube is provided with an anti-reflection, anti-electrostatic charge film on the outer surface of the glass panel 1 for preventing the reflection of external light incident upon the glass panel portion or the image display screen from being reflected thereby, to prevent deterioration of the contrast of the image display or for preventing the glass panel portion from being charged with static electricity.
FIG. 12 is a section view showing, on an enlarged scale, a portion A of the glass panel portion of FIG. 11 for explaining one example of an external light anti-reflection structure of the cathode ray tube. In FIG. 12, reference numeral 42 designates a black matrix; numeral 43 denotes a phosphor screen; numeral 44 denotes a metal back; numeral 51 denotes an electron beam passing opening of the shadow mask; symbols R, G and B denote the trajectories of electron beams of individual colors; numeral 20 denotes an anti-reflection, anti-electrostatic charge film; numeral 23 denotes light emitted from the phosphor screen; numeral 24 denotes external light incident on the glass panel of the cathode ray tube; and numerals 25 and 26 denote reflected external light. The same reference numerals as those of FIG. 11 designate identical elements.
In FIG. 12, the three electron beams (R, G and B), emitted from the electron guns, are subjected to color selection for the individual phosphor dots 43 of the R, G and B colors by the electron beam passing opening 51 of the shadow mask 5 to cause them to impinge upon the proper color dots of the phosphor screen 4.
The phosphor dots 43 are excited by the impingement of the electron beams to emit light, which passes through the glass panel portion 1. The anti-reflection, anti-electrostatic charge film 20 is formed on the outer surface of the glass panel portion. The external light 25 which reaches the anti-reflection, anti-electrostatic charge film 20 of the glass panel portion 1 is suppressed in light energy through absorption or interference in the anti-reflection, anti-electrostatic charge film 20, so that normal reflection of this light toward the outer surface of the film 20 is prevented together with diffusion of reflected light 26 by the surface of the anti-reflection, anti-electrostatic charge film 20.
This anti-reflection, anti-electrostatic charge film is formed by one of various methods, but generally it is formed by the so-called xe2x80x9csol-gel-method.xe2x80x9d
Specifically, there is disclosed in Japanese Patent Laid-Open No. 334853/1992 a method of forming a two-layered anti-reflection, anti-electrostatic charge film by forming a film of a mixed composition in which ultra fine particles (having a diameter no more than several tens of nm) of a conductive oxide (e.g., A.T.O.: tin oxide containing antimony oxide, or I.T.O.: indium oxide containing tin oxide) for forming a high refractive index film are dispersed in an alcoholic solution, by so-called xe2x80x9cspin-coatingxe2x80x9d to form a flat lower film having a thickness of about 60 to 100 nm, and by spin- or spray-coating the underlying film with a hydrolysate solution of silicon alkoxide to form a flat upper film having a thickness of 80 to 130 nm.
There is also disclosed in Japanese Patent Laid-Open No. 343008/1993 a method in which a film of an organic or inorganic tin compound containing antimony is formed on the glass panel of a cathode ray tube by chemical vapor deposition (hereinafter abbreviated to CVD) to form an A.T.O. film having a high refractive index, the A.T.O. film is coated flatly with a hydrolysate solution of silicon alkoxide of a thickness of 80 to 100 nm to form a film having a low refractive index, the second-layer film is spray-coated with the hydrolysate solution of silicon alkoxide to a thickness of 10 to 50 nm to form a third-layer scattering film having a low refractive index, so as to reduce the density of the reflected color exhibited by the second-layer of the anti-reflection, anti-electrostatic charge film and the reflectance in the human visible region of 400 to 700 nm, and the third-layer film is made uneven.
In the processes described above, the structure, in which a low refractive index film is formed over a high refractive index are individually made flat, is made substantially identical to the theoretical one for the two-layered anti-reflection film (described on pp. 100 to 103, OPTICAL THIN FILM written by Kozo Ishiguro et al., 1986, KYORITSU SHUPPAN). As a result, the structure has a V-shaped reflection characteristic, in the form of a reflection spectrum in which the reflectances at the two wavelengths at the ends of the visible region or 400 to 700 nm are higher than that at the central wavelength.
When the reflectance in the visible region is lowered, therefore, the reflectances at the two wavelengths at the ends are higher than that at the central wavelength. As a result, the color of the reflected light, i.e., the reflection color is intensified, and the reflectance is raised when the reflection color is reduced. In order to diminish this undesirable effect, a third-layer film in the form of an uneven film having a small thickness and a low refractive index is used. However, this effect is not sufficient when the height of the unevenness is small and the density thereof is high, namely, when the number of projections and recesses per unit area is large. On the other hand, when the height of the unevenness is large and the density thereof is high, the intensity of the scattering is increased to lower the resolution of the cathode ray tube.
Since a high refractive index film is formed by spin-coating or CVD processes, there arises a problem in that the process is complicated, resulting in an increase in the cost of manufacture.
An object of the present invention is to solve the aforementioned problems of the prior art and to provide a cathode ray tube having an anti-reflection, anti-electrostatic charge film which prevents the reflection of external light on a glass panel portion thereof, resulting in an increase in the contrast, while preventing formation of an electrostatic charge.
In a cathode ray tube of the present invention, a multi-layered anti-reflection, anti-electrostatic charge film formed on the outer face of the glass panel includes a high refractive index film having a refractive index of 1.6 to 2.2 and a low refractive index film having a refractive index of 1.3 to 1.58. The high refractive index film is sandwiched between the outer face of the glass panel and the low refractive index film, and an unevenness having an average diameter of 5 to 80 xcexcm is formed at the interface between the high refractive index film and the low refractive index film. The interface has a height of 10 to 40 nm. The unevenness of the outer surface of the low refractive index film is smaller than the average roughness Rz of the unevenness of the interface between the high refractive index film and the low refractive index film, or the outer surface of the low refractive index film is flat.
In the cathode ray tube of the present invention, moreover, the high refractive index film and the low refractive index film are made of anti-reflection, anti-electrostatic charge films which are formed by a spray-coating step, followed by a spin-coating step or a spray-coating step, and then a spray-coating step in this order.
According to a first aspect of the present invention, there is provided a cathode ray tube comprising a vacuum enclosure including a glass panel whose inner face is coated with a phosphor film to form a screen, a neck portion housing electron guns, and a funnel portion connecting the glass panel and the neck portion, wherein a high refractive index film (of a refractive index of 1.6 to 2.2) and a low refractive index film (of a refractive index of 1.3 to 1.58) are formed on the outer face of the glass panel, with the high refractive index film being sandwiched between the outer face of the panel glass and the low refractive index film, and an unevenness having an average diameter of 5 to 80 xcexcm and a height of 10 to 40 nm is provided at the interface between the high refractive index film and the low refractive index film.
According to a second aspect of the present invention, moreover, the outer surface of the low refractive index film is flattened (the average roughness Rz being no more than 10 nm).
With this type of construction, the characteristic curve of reflection is flattened to lower the average reflectance in the range of 400 to 700 nm and the dependence of the intensity of the reflected light on the wavelength is weakened to improve the image clarity of the cathode ray tube.
According a third aspect of the present invention, the low refractive index film has an average roughness Rz of more than 10 nm on its outer surface. Thanks to this construction, the image clarity of the cathode ray tube is improved even further by the diffuse reflection of external light from the low refractive index film.
According to a fourth aspect of the present invention, moreover, the average roughness Rz of the outer surface of the low refractive index film is smaller than the average roughness Rz of the unevenness of the interface between the high refractive index film and the low refractive index film. Thanks to this construction, the image clarity of the cathode ray tube is also improved by the diffuse reflection of external light from the low refractive index film. Here, the image clarity of the cathode ray tube is improved even more if the average roughness Rz of the unevenness of the outer surface of the low refractive index film and the number of projections and recesses per unit area are smaller than those of the interface between the low refractive index film and the high refractive index film.
According to a fifth aspect of the present invention, moreover, the material for forming the high refractive index film contains particles of a conductive oxide or metal, and the material for forming the low refractive index film contains a silicon compound or a fluorine compound such as MgF2 or CaF2. Thanks to this construction, the dependence of the intensity of the reflected light on the temperature is weakened so as to flatten the reflection characteristic curve, and the density of the reflected color is lowered to improve the image clarity of the cathode ray tube. Here, the particles of the conductive oxide or metal contained in the high refractive index film may be so-called ultra fine particles having an average diameter less than 70 nm.
By employing the so-called xe2x80x9csol-gel-methodxe2x80x9d, according to the present invention, the high refractive index film layer of the anti-reflection, anti-electrostatic charge film, having two layers, basically is given a structure in which the interface between the high refractive index film and the low refractive index film on the side of the high refractive index opposite to the glass panel plate is made uneven, so that the density of the reflected light, which is a defect of the two-layered anti-reflection, anti-electrostatic charge film of the prior art, is lowered to flatten the reflection curve. As a result, it is possible to provide a display device, such as a cathode ray tube which can have a lowered average reflectance in the range of 400 to 700 nm and which can have less light scattering, thereby improving the contrast of the display screen and the image clarity.
According to the present invention, moreover, the high refractive index film can be formed by spray-coating to reduce the amount of expensive solution used in the process, whereby the manufacturing process is simplified and the maintenance cost of the manufacturing facility is lowered.